Re: [PATCH 4/6] io_uring: let fast poll support multishot

[Pavel Begunkov <asml.silence@gmail.com>]

On 9/3/21 12:00 PM, Hao Xu wrote:
> For operations like accept, multishot is a useful feature, since we can
> reduce a number of accept sqe. Let's integrate it to fast poll, it may
> be good for other operations in the future.

__io_arm_poll_handler()         |
  -> vfs_poll()                 |
                                | io_async_task_func() // post CQE
                                | ...
                                | do_apoll_rewait();
  -> continues after vfs_poll(),|
     removing poll->head of     |
     the second poll attempt.   |


One of the reasons for forbidding multiple apoll's is that it
might be racy. I haven't looked into this implementation, but
we should check if there will be problems from that.

FWIW, putting aside this patchset, the poll/apoll is not in
the best shape and can use some refactoring.


> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
> ---
>  fs/io_uring.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index d6df60c4cdb9..dae7044e0c24 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -5277,8 +5277,15 @@ static void io_async_task_func(struct io_kiocb *req, bool *locked)
>  		return;
>  	}
>  
> -	hash_del(&req->hash_node);
> -	io_poll_remove_double(req);
> +	if (READ_ONCE(apoll->poll.canceled))
> +		apoll->poll.events |= EPOLLONESHOT;
> +	if (apoll->poll.events & EPOLLONESHOT) {
> +		hash_del(&req->hash_node);
> +		io_poll_remove_double(req);
> +	} else {
> +		add_wait_queue(apoll->poll.head, &apoll->poll.wait);
> +	}
> +
>  	spin_unlock(&ctx->completion_lock);
>  
>  	if (!READ_ONCE(apoll->poll.canceled))
> @@ -5366,7 +5373,7 @@ static int io_arm_poll_handler(struct io_kiocb *req)
>  	struct io_ring_ctx *ctx = req->ctx;
>  	struct async_poll *apoll;
>  	struct io_poll_table ipt;
> -	__poll_t ret, mask = EPOLLONESHOT | POLLERR | POLLPRI;
> +	__poll_t ret, mask = POLLERR | POLLPRI;
>  	int rw;
>  
>  	if (!req->file || !file_can_poll(req->file))
> @@ -5388,6 +5395,8 @@ static int io_arm_poll_handler(struct io_kiocb *req)
>  		rw = WRITE;
>  		mask |= POLLOUT | POLLWRNORM;
>  	}
> +	if (!(req->flags & REQ_F_APOLL_MULTISHOT))
> +		mask |= EPOLLONESHOT;
>  
>  	/* if we can't nonblock try, then no point in arming a poll handler */
>  	if (!io_file_supports_nowait(req, rw))
> 

-- 
Pavel Begunkov